I'm trying to make a quiz to help revise for a module at uni and have come across this in the revision notes, anybody fancy a guess?.
An Internet banking system allows customers to select their own 6-digit
security number. Two digits are selected at random by the bank for the
client to enter as an authentication mechanism each time the customer
logs in. Assuming an eavesdropper is able to read the digits entered by
the customer what is the least possible number of logins the
eavesdropper would have to see in order to learn the security number?